experiencing a similar issue when trying to import my own network shape-file. Going through the import tool that lets me use simplify streets the finish button is grayed out but if i just drag and drop the .shp into the 3d view it loads up fine

When the finish button is deactivated usually ther is a wrong or missing input. Is there any error or hint shown on the first page the greyed out finish button appears? The hint is usually shown below the tiltle of the dialog.